Lithium secondary cells and methods for preparing active materials for negative electrodes

ABSTRACT

A lithium secondary cell wherein a carbon material treated with a fluorinating agent and having substantially no C--F covalent bond, is used as an active material for negative electrode, provides a secondary cell having improved cell life. Further, a lithium secondary cell wherein a carbon material treated with a fluorinating agent and having an amount of fluorine extractable with an aqueous alkaline solution being at most 0.05 wt %, is used as an active material for negative electrode, or a lithium secondary cell wherein a carbon material treated with a fluorinating agent and containing lithium, is used as an active material for negative electrode, provides a secondary cell which is free from a problem of expansion of the casing during charging due to fluorine remaining in a small amount in the cell.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to lithium secondary cells and methods forpreparing active materials for negative electrodes for the lithiumsecondary cells.

2. Discussion of Background

With rapid progress of electronic technology in recent years, trend forsmall sizes and light weights of electronic appliances is remarkable,and expectation for cells as their power sources is likewise high.Namely, there is substantial expectation for cells provided withexcellent storage stability as well as high energy density with smallsizes and light weights.

As the most prospective cell system to satisfy such expectation, aresearch has been active for developing lithium cells wherein lithium isused as the negative electrode. By virtue of their excellent properties,lithium secondary cells have been rapidly put on market as portablepower sources for e.g. VTR cameras, portable telephones or personalcomputers. However, such cells usually have a difficulty that noadequate charge-discharge cycle life can be obtained.

Therefore, it has been proposed to use, as negative electrode materials,carbon-lithium intercalation compounds having lithium ions intercalatedto various carbon materials, and various carbon materials suitable forthis purpose have been studied.

For example, it has been proposed to use, as a negative electrodematerial, graphite such as natural graphite, or a carbon materialprepared by baking various resins or pitches. However, a furtherimprovement in the cell performance is still desired to make itapplicable to a large size cell useful for a so-called dispersion typepower storage system such as a system for domestic power storage atnight or for electric vehicle.

SUMMARY OF THE INVENTION

Under these circumstances, the present inventors have conducted variousstudies with an aim to obtain a negative electrode material for alithium secondary cell, which makes it possible to further prolong thecell life, and as a result, have arrived at a first aspect of thepresent invention.

Whereas, a small amount of fluorine remains in the carbon negativeelectrode material treated by the first aspect of the present invention,whereby a problem sometimes occurs such that after preparation of acell, such fluorine evaporates during charging, so that the casing ofthe cell is likely to be expanded. If this phenomenon occurs, the liquidsurface of the electrolyte lowers, which brings about not only adecrease in capacitance but also a serious problem in safety.Accordingly, solution of this problem is desired.

Accordingly, the present inventors have conducted various studies withan aim to remove fluorine remaining in the carbon negative electrodematerial treated with the fluorinating agent and as a result, havearrived at second and third aspects of the present invention.

That is, in the first aspect, the present invention provides a lithiumsecondary cell, wherein a carbon material treated with a fluorinatingagent and having substantially no C--F covalent bond, is used as anactive material for negative electrode, and a method for preparing anactive material for negative electrode for a lithium secondary cell,which comprises subjecting a surface of a carbon material tofluorine-containing gas treatment by contacting a fluorine-containinggas thereto at a temperature of from 0 to 30° C. for from 1 minute to 2days.

In the second aspect, the present invention provides a lithium secondarycell, wherein a carbon material treated with a fluorinating agent andhaving an amount of fluorine extractable with an aqueous alkalinesolution being at most 0.05 wt %, is used as an active material fornegative electrode, and a method for preparing an active material fornegative electrode for a lithium secondary cell, which comprisestreating a carbon material with a fluorinating agent and then contactingit with a steam-containing gas in a heated atmosphere.

In the third aspect, the present invention provides a lithium secondarycell, wherein a carbon material treated with a fluorinating agent andcontaining lithium, is used as an active material for negativeelectrode, and a method for preparing an active material for negativeelectrode for a lithium secondary cell, which comprises treating acarbon material with a fluorinating agent and then adding lithium metaland/or a lithium compound other than lithium fluoride, to the carbonmaterial.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Firstly, the carbon material to be used in the present invention may,for example, be coke prepared from various pitches as startingmaterials, a resin-type carbon material obtained by baking e.g. a phenolresin, or natural or artificial graphite. Coke is preferably employed.

Such coke may be the one prepared by subjecting petroleum based heavyoil such as a FCC (fluidized catalytic cracking) residual oil, an EREoil (a by-product oil from ethylene production), a normal pressureresidual oil or a reduced pressure residual oil, or a coal tar basedcarbon material such as coal tar or coal tar pitch, to delayed coking ata temperature of from 400 to 500° C., followed by baking at atemperature of from 1,000 to 1,500° C.

The carbon material is used usually in the form of particles. Theparticle size is not particularly limited so long as it is not too largeto be used for an electrode for a cell.

In the present invention, such a carbon material is treated with afluorinating agent, usually in the particle form, or in some cases,after being formed into a negative electrode shape. As the fluorinatingagent, a substance which supplies fluorine atoms, such as fluorine gas,may be employed.

The treating temperature with the fluorinating agent is selected withina temperature range in which substantially no C--F covalent bond will beformed, i.e. from -100° C. to 90° C., preferably from 0 to 30° C. Thetreating time with the fluorinating agent may be short so long asfluorine is uniformly supplied to the carbon material, and even if thetime is prolonged, no corresponding effects can be expected. Therefore,the treating time is usually from 1 minute to 2 days, preferably notlonger than 30 minutes. The fluorine partial pressure of fluorine gas isnot particularly limited and may be under elevated or reduced pressure.However, the treatment is preferably conducted under a pressure of from1 mmHg to 100 mmHg.

The fluorine gas or the like may be used alone or may be used as mixedwith an inert gas such as argon, or with oxygen gas or the like. It isparticularly preferred to use it as mixed with oxygen gas.

The fluorinated carbon material thus obtained has the followingproperties. Namely, it does not have a distinct peak at from 1,000 to1,200 kayser attributable to a C--F bond by the FT-IR measurement. Ofcourse, the C--F bond may partially be formed, but it must be at most 1%.

In the second aspect of the present invention, the amount of fluorineextractable with an aqueous alkaline solution is reduced to a level ofat most 0.05 wt % by subjecting the above fluorinated carbon material totreatment for removal of alkali extractable fluorine.

The treatment for removal of alkali extractable fluorine may be carriedout to the above fluorinated carbon material prior to forming it into anegative electrode material.

The treatment for removal of alkali extractable fluorine is carried outby contacting the fluorinated carbon material with a steam-containinggas in a heated atmosphere. The steam-containing gas may usually be airand/or an inert gas such as nitrogen or argon, which contains from 1 to60 vol %, preferably from 10 to 40 vol %, of steam. The flow rate of thesteam-containing gas is usually from 50 to 1,000 ml/min, preferably from200 to 300 ml/min, per 50 g of the carbon material. The temperature forthe treatment is usually from 150 to 350° C., preferably from 200 to300° C. The treating time is usually from a few minutes to a few tenshours, preferably from 1 to 10 hours. The treating apparatus is notparticularly limited so long as it is thereby possible to carry out thecontact of the carbon material with the steam-containing gassatisfactorily. However, it is preferred to use the same apparatus asused for the fluorinating treatment, to conduct the steam treatmentfollowing the fluorinating treatment.

The amount of fluorine extractable with an aqueous alkaline solutionafter the fluorinating treatment prior to the treatment for removal ofthe alkali extractable fluorine, varies also depending upon thefluorinating treatment conditions, but it is usually at a level of 0.1wt %. In the lithium secondary cell according to the second aspect ofthe present invention, a carbon material having such an alkaliextractable fluorine removed to a level of at most 0.05 wt %, is used asan active material for negative electrode. Here, the amount of fluorineextractable with an aqueous alkaline solution (hereinafter referred toas alkali extractable fluorine) can be determined by a method wherein asample is added to an aqueous alkaline solution such as an aqueoussodium hydroxide solution or an aqueous potassium hydroxide solutionhaving a concentration of from 0.1 to 1 N, followed by sufficientextraction, whereupon the concentration of fluorine ions in the filtratepassed through a filter, is measured by ion chromatography.

On the other hand, in the third aspect of the present invention, thefluorinated carbon material is subjected to treatment for fixing thealkali extractable fluorine to the carbon material. The treatment forfixing the alkali extractable fluorine may be carried out to the abovefluorinated carbon material either before or after forming it into anegative electrode material. By such fixing treatment, the amount offluorine extractable with the above aqueous alkaline solution is reducedpreferably to a level of at most 0.05 wt %.

In the third aspect of the present invention, the treatment for fixingthe alkali extractable fluorine is carried out by adding lithium metaland/or a lithium compound other than lithium fluoride, to the carbonmaterial. In a case where a powder of lithium metal is added to thecarbon material, a particulate carbon material and a powder of lithiummetal may be mixed. In a case where a lithium compound other thanlithium fluoride is to be added to the carbon material, the carbonmaterial may be dipped in a dispersion or solution having the lithiumcompound other than lithium fluoride, such as lithium hydroxide,dispersed or dissolved in a lower boiling point solvent such as water oralcohol, or such a dispersion or solution may be sprayed to the carbonmaterial.

The amount of lithium in the lithium metal and/or the lithium compoundother than lithium fluoride, to be added, may be at a level of 1.5 timesin reaction equivalent, to the amount of the alkali extractable fluorinein the carbon material prior to the fixing treatment. After theaddition, the solvent may be removed, for example, by heating, as thecase requires.

In the lithium secondary cell according to the third aspect of thepresent invention, a carbon material containing lithium, which has beentreated with a fluorinating agent, followed by the treatment for fixingthe alkali extractable fluorine, as mentioned above, is used as anactive material for negative electrode.

The treated carbon material obtained by the above treatment, may bemixed as an active material for negative electrode, with a binder or thelike, by a conventional method, to obtain a negative electrode.

A positive electrode and an electrolyte having an electrolyte substancedissolved in a non-aqueous solvent, may be those commonly used fornon-water type secondary cells heretofore, and they are not particularlylimited.

Specifically, for the positive electrode, LiCoO₂, MnO₂, TiS₂, FeS₂, Nb₃S₄, Mo₃ S₄, Mo₃ Se₄, CoS₂, V₂ O₅, P₂ O₃, CrO₃, V₃ O₈, TeO₂ or GeO₂ may,for example, be used. As the electrolyte substance, LiClO₄, LiBF₄ orLiPF₆ may, for example, be employed. The non-aqueous solvent for such anelectrolyte may, for example, be propylene carbonate, tetrahydrofuran,1,2-dimethoxyethane, dimethylsulfoxide, dioxorane, dimethylformamide,dimethylacetamide, or a solvent mixture of two or more of them.

As a separator, a porous material is preferred in order to reduce theinternal resistance of the cell. For example, a non-woven fabric of e.g.polypropylene, or a separator made of a organic solvent resistantmaterial such as glass filter, is preferably employed.

The negative electrode, the positive electrode, the electrolyte and theseparator may be assembled into a secondary cell by a common method, forexample by assembling them in a cell casing made of e.g. stainlesssteel, nickel-plated stainless steel or polypropylene. The cellstructure may be a spiral structure in which strip-shaped positive andnegative electrodes are spirally wound with a separator interposedtherebetween. Otherwise, it is possible to employ a method wherein apellet-like positive electrode and a disk-shaped negative electrode areinserted with a separator interposed therebetween, in a button-shapedcasing. When it is applied to a large size battery, it is common toemploy a so-called angular battery structure wherein a plurality ofcells are laminated.

Now, the present invention will be described in further detail withreference to Examples, but it should be understood that the presentinvention is by no means restricted to such specific Examples.

EXAMPLE 1

Ethylene bottom oil (EHE) with BMCI (Bureau or Mines Correlation Index:showing the fractional properties of the material)=92.8, elementalanalytical value=92.8 wt % and H=7.2 wt %, was put into a 4 l autoclavein an amount of 3.5 kg, and the temperature was raised from roomtemperature to 350° C. in one hour and then maintained at 350° C. forone hour. Thereafter, the temperature was raised to 480° C. at a rate of10° C./hr and further maintained at 480° C. for 30 hours. After coolingto room temperature, the carbon material was taken out from theautoclave. Then, the carbon material of block-shape taken out from theautoclave, was pulverized by a jet mill to a particle size of from 10 to44 μm, and the pulverized material was heated in a porcelain to 1,200°C. at a temperature raising rate of 300° C./hr to obtain a coke of thepresent invention. The elemental analytical values of the coke are shownin Table 1.

This coke was mixed with a binder and formed into a block, followed bybaking. Then, the block-shaped product was cut into a size of a workingelectrode, and the product was put into a reactor for fluorinationtreatment. While maintaining the interior of the reactor in a vacuumedcondition, a fluorine-containing gas (F₂ :8 vol %, N₂ :92 vol %) wasintroduced, and fluorination treatment was carried out under atmosphericpressure for 10 minutes.

After the fluorination, the product was once washed to remove F₂adsorbed on the surface and then thoroughly dried at 50° C.

Using the obtained negative electrode as a working electrode and usinglithium metal for a counter electrode and a reference electrode, lithiumwas occluded in the negative electrode until the potential became 0 V.Using electrolytic manganese dioxide as a positive electrode andpropylene carbonate having LiClO₄ dissolved in a concentration of 1mol/l therein, as an electrolyte, a lithium secondary cell was preparedby a usual method.

Then, the discharge characteristics of the lithium secondary cell weremeasured.

The measurement was carried out under a constant current charge anddischarge of 50 mA/g (negative electrode carbon base), and the servicecapacity was a capacity until the cell voltage dropped to 2.0 V. As areference, measurement was carried out under the same condition withrespect to a conventional lithium secondary cell wherein a coke obtainedin the same manner as above except that no fluorination treatment wasapplied, was used. The results are shown in Table 2.

As is evident from Table 2, as compared with the conventional lithiumsecondary cell wherein a carbon negative electrode which was subjectedto no fluorination treatment, was used, the lithium secondary cell ofthe present invention wherein the coke subjected to fluorinationtreatment was used, had a remarkably large service capacity, as thewettability with the electrolyte was improved.

                  TABLE 1                                                         ______________________________________                                               O/C   0.0011                                                                  S/C   <0.0003                                                                 N/C   <0.0026                                                                 C wt %                                                                              99.84                                                            ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                                       Service capacity                                                              (Ah/kg)                                                        Carbon material                                                                              First cycle                                                    ______________________________________                                        Fluorinated coke                                                                             300                                                            Non-treated coke                                                                             250                                                            ______________________________________                                    

EXAMPLE 2

Green coke having a volatile component of 5.8 wt % obtained by cokingcoal tar in a coke drum at a temperature of from 450 to 500° C. for 24hours, was finely pulverized by a jet mill to obtain a fine powder ofgreen coke having an average particle size of 8.8 μm and a maximumparticle size of at most 35 μm. This fine powder of green coke wasplaced in a tray made of graphite and heated to 1,200° C. at atemperature raising rate of about 10° C./min in a nitrogen atmosphere ina box-shaped electric furnace and maintained for two hours to obtain acarbon material.

The carbon material was put into a 10 l reactor. While maintaining theinterior of the reactor in a vacuumed condition, a fluorine-containinggas (F₂ :20 mmHg, O₂ :660 mmHg) was introduced, and fluorinationtreatment was carried out at room temperature under atmospheric pressurefor 30 minutes. After the fluorination treatment, the residual gas wasremoved from the reactor, and then the temperature was raised to 200°C., and treatment was carried out for 2 hours while introducing 600ml/min of air containing 25 vol % of saturated steam.

The amount of alkali extractable fluorine in the carbon material thusobtained, was measured by means of a 1 N sodium hydroxide aqueoussolution and found to be 0.02 wt %, which was substantially lower than0.1 wt % in the carbon material which was not subjected to the fluorineremoval treatment.

This carbon material was mixed with polyvinylidene fluoride (PVDF) anddimethylformamide (DMF) to obtain a paste, which was coated on a metalplate and dried and pressed to obtain a negative electrode.

On the other hand, for a positive electrode, LiCoO₂ was mixed withacetylene black, PVDF and DMF to obtain a paste, which was coated on ametal plate and dried and pressed to obtain a positive electrode.

The positive electrode and the negative electrode were insulated with aseparator of polyolefin.

As an electrolyte, the one prepared by dissolving LiPF₆ in a solventmixture of propylene carbonate (PC) and diethyl carbonate, was used.

These materials were set in a cell casing made of polypropylene with awidth of 70 mm, and sealed to obtain a lithium secondary cell.

This cell was subjected to charging and discharging at a constantcurrent of 30 A, whereby no expansion of the cell casing was observedeven after completion of the charge-discharge of 10 cycles. The resultsare shown in Table 3.

COMPARATIVE EXAMPLE 1

A lithium secondary cell was prepared in the same manner as in Example 2except that the carbon material subjected to the fluorination treatmentin the same manner as in Example 2, was used directly as a carbonmaterial for negative electrode without carrying out the fluorineremoval treatment.

This cell was subjected to charging and discharging repeatedly in thesame manner as in Example 2, whereby the initial width of 70 mm of thecell casing expanded to 81.5 mm by seven cycles. Consequently, theliquid surface of the electrolyte lowered substantially. The results areshown in Table 3.

COMPARATIVE EXAMPLE 2

The carbon material subjected to fluorination treatment in the samemanner as in Example 2, was treated at 200° C. for 2 hours whileintroducing air containing no steam at a rate of 600 ml/min.

The amount of alkali extractable fluorine in the obtained carbonmaterial was measured and found to be 0.09 wt %, which was substantiallythe same as 0.1 wt % before the treatment. A lithium secondary cell wasprepared in the same manner as in Example 2 except that this carbonmaterial was used as the carbon material for negative electrode.

This cell was subjected to charging and discharging repeatedly in thesame manner as in Example 2, whereby the initial width of 70 mm of thecell casing expanded to 81.0 mm by 7 cycles. Consequently, the liquidsurface of the electrolyte lowered substantially. The results are shownin Table 3.

                  TABLE 3                                                         ______________________________________                                                            F content                                                                              Expansion                                                            in treated                                                                             (mm) of                                                              product  cell                                                      Treating method                                                                          (wt %)   casing                                           ______________________________________                                        Example 2  Treatment with a                                                                           0.02     0                                                       steam-containing                                                              gas                                                                Comparative                                                                              No treatment 0.10     11.5                                         Example 1                                                                     Comparative                                                                              Treatment with a                                                                           0.09     11.0                                         Example 2  gas containing no                                                             steam                                                              ______________________________________                                    

EXAMPLE 3

Green coke having a volatile content of 5.8% obtained by coking coal tarin a coke drum at a temperature of from 450 to 500° C. for 24 hours, wasfinely pulverized by a jet mill to obtain a fine powder of green cokehaving an average particle size of 8.8 μm and a maximum particle size ofat most 35 μm. This fine powder of green coke was placed in a tray madeof graphite and heated to 1,200° C. at a temperature raising rate ofabout 10° C./min in a nitrogen atmosphere in a box-shaped electricfurnace and maintained for two hours to obtain a carbon material.

The carbon material was put into a 10 l reactor, and while maintainingthe interior of the reactor under a vacuumed condition, afluorine-containing gas (F₂ :20 mmHg, O₂ :660 mmHg) was introduced, andfluorination treatment was carried out at room temperature underatmospheric pressure for 30 minutes.

To the fluorinated carbon material, a saturated aqueous solution oflithium hydroxide (LiOH) was sprayed, and then water was removed in adryer. The amount of LiOH to the carbon material was adjusted so that itwould be 1.5 times in reaction molar amount to the amount of alkaliextractable fluorine in the carbon material. The carbon material afterdrying was examined by X-ray diffraction, whereby a diffraction line ofLiF was observed. The amount of alkali extractable fluorine in theobtained carbon material was measured by means of a 1 N sodium hydroxideaqueous solution and found to be 0 wt %.

This carbon material was mixed with polyvinylidene fluoride (PVDF) anddimethylformamide (DMF) to obtain a paste, which was coated on a metalplate and dried and pressed to obtain a negative electrode.

On the other hand, for a positive electrode, LiCoO₂ was mixed withacetylene black, PVDF and DMF to obtain a paste, which was coated on ametal plate and dried and pressed to obtain a positive electrode.

The positive electrode and the negative electrode were insulated by aseparator of polyolefin.

As the electrolyte, the one having LiPF₆ dissolved in a solvent mixtureof propylene carbonate (PC) and diethyl carbonate, was used.

These materials were set in a cell casing made of polypropylene with awidth of 70 mm and sealed to obtain a lithium secondary cell.

This cell was subjected to charging and discharging repeatedly at aconstant current of 30 A, whereby no expansion of the cell casing wasobserved even after completion of the charge and discharge of 10 cycles.The results are shown in Table 4.

COMPARATIVE EXAMPLE 3

A lithium secondary cell was prepared in the same manner as in Example 3except that the carbon material subjected to fluorination treatment inthe same manner as in Example 3, was used directly as a carbon materialfor negative electrode without carrying out fluorine fixing treatment.

This cell was subjected to charging and discharging repeatedly in thesame manner as in Example 3, whereby the initial width of 70 mm of thecell casing expanded to 81.5 mm. Consequently, the liquid surface of theelectrolyte lowered substantially. The results are shown in Table 4.

COMPARATIVE EXAMPLE 4

To the carbon material subjected to fluorination treatment in the samemanner as in Example 3, only water was sprayed in the same amount as theLiOH saturated aqueous solution in Example 3, and then water was removedin a drier.

A lithium secondary cell was prepared in the same manner as in Example 3except that the obtained carbon material was used as a carbon materialfor negative 5 electrode.

This cell was subjected to charging and discharging repeatedly in thesame manner as in Example 3, whereby the initial width of 70 mm of thecell casing expanded to 80.0 mm. Consequently, the liquid surface of theelectrolyte lowered substantially. The results are shown in Table 4.

                  TABLE 4                                                         ______________________________________                                                             F content                                                                              Expansion                                                            in treated                                                                             (mm) of                                                              product  cell                                                     Treating method                                                                           (wt%)    casing                                          ______________________________________                                        Example 3  Treatment with a                                                                            0        0                                                      aqueous lithium                                                               hydroxide solution                                                 Comparative                                                                              No treatment  0.10     11.5                                        Example 3                                                                     Comparative                                                                              Treatment with                                                                              0.09     11.0                                        Example 4  water containing                                                              no lithium                                                                    compound                                                           ______________________________________                                    

According to the present invention, it is possible to obtain a lithiumsecondary cell having an improved cell life, which is industrially veryuseful.

What is claimed is:
 1. A lithium secondary cell, comprising a negativeelectrode comprising, as an active material:a carbon material containingfluorine, wherein at most 0.05 wt. % fluorine is extractable from saidcarbon material with an aqueous alkaline solution, and said carbonmaterial has no distinct peak at 1,000 to 1,200 kayser by FT-IRmeasurement attributable to a C--F bond.